This invention relates to a magnetic disk to be loaded in a magnetic disk apparatus, such as a HDD (hard disk drive), a glass substrate for the magnetic disk, and methods of producing the magnetic disk and the glass substrate.
At present, following the rapid development of the IT industry, the information recording technology, in particular, the magnetic recording technology is requested to achieve dramatic technical innovation. For a magnetic disk to be loaded in a HDD (hard disk drive), it is required to develop a technology capable of achieving an information recording density of 40 Gbit/inch2 or more in response to a demand for a higher information capacity.
The magnetic disk is required to be excellent in magnetic characteristic in a flying/traveling direction of a magnetic head. To this end, there is known a technique of forming a concentric texture on a surface of a substrate for a magnetic disk to impart a circumferential magnetic anisotropy to the magnetic characteristic of the magnetic disk, thereby improving the magnetic characteristic as a magnetic recording medium to achieve a higher recording density as described in, for example, Japanese Patent Application Publication (JP-A) No. 2002-30275 (Hereinafter referred to as Reference 1).
Recently, attention is drawn to a glass substrate as a substrate for a magnetic disk, which is suitable for a higher recording density. The glass substrate is suitable for use with a magnetic head of a low flying height because a flat and smooth surface is obtained, and is advantageous in an improvement of a S/N ratio of a recording signal and an increase in recording density. Thus, the glass substrate is excellent in adaptability to a low flying height of the magnetic head.
As the glass substrate of the type, a glass substrate for a magnetic recording medium is described in, for example, Japanese Patent Application Publication (JP-A) No. 2002-32909 (hereinafter referred to as Reference 2) is known.
In the above-mentioned conventional techniques, the substrate for a magnetic disk is provided with the concentric texture so as to improve the magnetic characteristic and a read/write characteristic of the magnetic disk and to contribute to an increase in information capacity.
In a magnetic disk apparatus such as a hard disk drive (HDD), use has been made of a CSS (Contact Start and Stop) system in which a magnetic head is kept in contact with a contact sliding zone (CSS zone) formed on a surface of a magnetic disk in a stopped state while, in a starting operation, the magnetic head slides in the CSS zone in contact with the disk surface to be floated up and, thereafter, a write or a read operation is carried out in a read/write zone of the disk surface outside the CSS zone. In an ending operation, the magnetic head is retreated from the read/write zone to the CSS zone and, thereafter, the magnetic head slides in the CSS zone in contact with the disk surface to be landed and stopped. In the CSS system, the starting operation and the ending operation in which contact sliding movement occurs will be called a CSS operation.
In the magnetic disk of the CSS system, it is necessary to provide both the CSS zone and the read/write zone on the disk surface. In addition, in order to avoid the magnetic head and the magnetic disk from being attracted or stuck to each other when they are contacted, it is necessary to provide the surface of the magnetic disk with a convex/concave shape having a predetermined surface roughness.
As a recent magnetic disk apparatus, a magnetic disk apparatus of a LUL (Load Unload) system is introduced instead of the CSS system which has been used so far. In the LUL system, a magnetic head is retreated on an inclined support, called a ramp, located outside a magnetic disk in a stopped state. In a starting operation, the magnetic head slides from the ramp to the magnetic disk after the rotation of the magnetic disk is started, and flies and travels over the magnetic disk to carry out a write or a read operation. In a stopping operation, the magnetic head is retreated to the ramp outside the magnetic disk and, thereafter, the rotation of the magnetic disk is stopped. A series of the above-mentioned operations will be called a LUL operation.
The LUL system does not require the CSS zone (contact sliding zone for the magnetic head) to be formed on the magnetic disk so that a wide read/write zone is secured on the surface of the magnetic disk as compared with the magnetic disk of the CSS system. Therefore, this system contributes to a higher recording capacity.
In the LUL system, the magnetic head is not brought into contact with the surface of the magnetic disk, unlike the CSS system. Therefore, it is unnecessary to provide the surface of the magnetic disk with the convex/concave shape for preventing attraction so that the surface of the magnetic disk can be extremely flattened and smoothed. Accordingly, with the magnetic disk of the LUL system, the flying height of the magnetic head can be remarkably lowered as compared with the CSS system. This results in an advantage that a high S/N ratio of the recording signal is achieved to contribute to a higher recording capacity of the magnetic disk apparatus.
Following recent introduction of the LUL system, the flying height of the magnetic head is discontinuously remarkably lowered. Consequently, the magnetic disk is required to stably operate even at a very small flying height of 10 nm or less. However, if the magnetic head flies and travels over the surface of the magnetic disk at such a very small flying height, there arises a problem of frequent occurrence of a fly stiction defect or a head corrosion defect.
The fly stiction defect is a defect in which the flying position or the flying height fluctuates while the magnetic head is flying and traveling, often followed by irregular variation in read output. Sometimes, the magnetic head is contacted with the magnetic disk during flying and traveling to cause a head crash defect, resulting in breakage of the magnetic disk. The head corrosion defect is a defect in which an element portion of the magnetic head is corroded to cause a trouble in the write or the read operation. Sometimes, the write or the read operation is impossible or the corroded element is expanded to damage the surface of the magnetic disk during flying and traveling. Occurrence of those defects will significantly deteriorate a HDI (Head Disk Interface) reliability of the magnetic head during flying and traveling, for example, a LUL durability, a CFT durability (Constant Flight durability), and a glide characteristic. Further, during flying and traveling, the magnetic head may fall onto the surface of the magnetic disk to be attracted thereto.
The present inventor systematically studied about those defects. As a result, it has been found out that those defects tend to frequently occur if an anisotropic texture (for example, a circumferential texture) for imparting a magnetic anisotropy to a magnetic layer of the magnetic disk is formed on a glass substrate. It is supposed that the anisotropic texture induces the above-mentioned defects related to the HDI reliability while the magnetic head is flying and traveling. Further, it has been found out that these defects tend to frequently occur, in particular, in an inner region of the disk, for example, on a principal surface of the disk in a region within a disk radius of 13 mm, especially, in a region within a disk radius of 12 mm.